1. Field of the Invention
The present invention relates to technologies for acquiring a photographed image in a wide range by using a photographing apparatus.
2. Related Background Art
A wide-angle photographing apparatus is known which can acquire a photographed image in a wide range by combining photographed images taken with a plurality of optical photographic systems. For example, this wide-angle photographing apparatus is applied to a monitor camera, a television conference camera and the like.
For a wide range photographing system, a method has been proposed by which a plurality of cameras are disposed radially as shown in FIG. 10 and photographed images taken with the cameras are joined together to realize paronama photographing. A camera 1 is disposed on each side of a polygonal housing 101 to photograph each divided area of a photographing range of 360°. The system of this kind is disclosed, for example, in Japanese Patent Application Laid-open Nos. H07-067020, H09-093471, No. 2001-204015 and the like.
Another system has also been proposed in which a hyperbolic mirror is used to photograph a whole peripheral area as shown in FIG. 11. One camera is disposed under a hyperbolic mirror 102 and a whole peripheral image is focussed on an image pickup device of the camera 1. An image focussed on the image pickup device will be described with reference to FIG. 12. In FIG. 12, an image in a whole peripheral area is focussed on a ring area 104 of an image pickup device 103. When the image is displayed on a monitor or the like, image processing is performed by an unrepresented processor circuit to convert the image in a laterally elongated panorama image. The system of this type is disclosed, for example, in Japanese Patent Application Laid-open No. H06-295333 and Japanese Patent Publication No. H09-505447 and the like.
A conventional system in which a plurality of cameras are disposed radially will be described with reference to FIG. 13.
In FIG. 13, three cameras 11, 12 and 13 are disposed radially to realize wide-angle photographing. The camera 11 is constituted of a photographing lens 2, an image pickup device 4 such as CCD and CMOS and a substrate 6, and a sector photographing field angle having a center at the principal point of the lens 2 on the object side is focussed on the image pickup device 4. The structure of the cameras 12 and 13 is similar to that of the camera 11, and the cameras are disposed so that the photograph field angles are overlapped. With the photographic field angles of the cameras 11 and 12, an object in the oblique line area surrounded by field angle boundaries 21 and 22 and their cross point 20 is taken into both the photographed images of the cameras 11 and 12.
Ideally, as shown in FIG. 14, if the principal point 3 of each camera on the object side is made coincident, an image taken with each camera has no parallax so that the images are contiguous with each other at each joining point and it is easy to join the images smoothly. However, in practice, each camera has a physical size and is interfered with each other. It is therefore difficult to dispose the cameras in such a manner that the principal points on the object side become coincident. From this reason, the radial layout such as shown in FIG. 13 has been used conventionally. Therefore, the principal points of adjacent cameras on the object side are spaced apart by a distance L shown in FIG. 13 (hereinafter this distance L is called a base line length).
With this layout, photographed images taken with adjacent cameras have a larger parallax difference as the base line length L becomes longer. Therefore, in an area where the joining portions are overlapped, the positions of an object in the adjacent photographed images differ considerably and the images in the joining portions are not coincident. It is therefore difficult to join the two images smoothly.
With reference to FIG. 15, description will be made on how an object is photographed in an area where the photographing field angles of adjacent cameras are overlapped. A nearest distance between the photographing ranges of cameras 11 and 12 is represented by S1. It is assumed that the cameras are disposed in such a manner that the boundary 21 of the photographing field angle of the camera 11 crosses the boundary 22 of the photographing field angle of the camera 12, at a cross point 20 at the nearest distance S1.
The photographed images to be taken with the cameras 11 and 12 are focused via lenses on the image pickup devices. The photographed images do not change even if it is virtually assumed that the image pickup plane exists on the object side with respect to the lens. Description will be made therefore assuming that the image pickup planes are at 23 and 24. An object 25 (white circle symbol) at the cross point 20 of the nearest distance S1 is photographed on the boundary between the image pickup planes 23 and 24 of the cameras 11 and 12. FIG. 16 shows that the photographed images 201 and 202 taken with the cameras 11 and 12 are juxtaposed in contact with each other. As shown in FIG. 16, an image 25a of the object 25 (white circle symbol) is photographed generally in unison on the boundary between the photographed images 201 and 202.
In FIG. 15, an object 26 (black triangle symbol) at a photographing distance S2 on the boundary 22 of the camera 12 is projected on the image pickup plane 23 of the camera 11 as indicated by an arrow in FIG. 15. Although not shown on the image pickup plane 24 of the camera 12, an object is projected on the boundary similar to the object 25. Therefore, in the photographed images shown in FIG. 16, although the photographed image 202 of the object 26 (black triangle symbol) is projected on the boundary as an image 26a, the photographed image 201 is projected as an image 26b shifted from the boundary. A shift amount 206 of the images taken with the cameras 11 and 12 is called a parallax amount.
An object 27 (white square symbol) at a infinite distance S3 on the boundary 22 of the camera 12 is projected on the image pickup plane 23 of the camera 11 at a position on a line parallel to the boundary 22 corresponding to the base line length L, as shown in FIG. 15. The object 27 is projected on the image pickup plane 24 of the camera 12 at the boundary similar to the objects 25 and 26. Therefore, in the photographed images shown in FIG. 16, the object 27 (white square symbol) at the infinite distance is photographed on the boundary as an image 27a in the photographed image 202, whereas it is photographed as an image 27b on a dotted line 204 in the photographed image 201. As the distance of an object becomes longer, a parallax amount 206 increases. The object at an infinite distance has a constant converged parallax amount represented by a distance between the dotted line 204 and the boundary 203 between the photographed images 201 and 201.
The foregoing description concerns about the object on the boundary 22 of the photographing field angle of the camera 12. Similar description can be made also for an object on the boundary 21 of the photographing field angle of the camera 11. Namely, as the parallax amount increases as the object distance becomes long, an object at an infinite distance is projected as an image on a dotted line 205 in the photographed image 202. An object in an area where the photographing field angles of the cameras 11 and 12 are overlapped, is duplicately photographed depending upon an object distance in the area sandwiched between the dotted lines 204 and 205, resulting in a parallax.
A parallax amount of an object at an infinite distance becomes large in proportion to the base line length L of the cameras 11 and 12. Therefore, if the base line length L is long, although the photographed images taken with the cameras 11 and 12 are coincident for an object at the nearest distance on the boundary, a parallax amount of an object at the infinite distance becomes large so that the photographed images are not coincident and smooth joining cannot be realized.
With such a radial layout, even if the cameras are disposed so as to shorten the base line length, the image pickup device substrates 6 of the cameras interfere with each other so that the lens front ends are made open. Therefore, the base line length L becomes long, a parallax difference between images taken with adjacent cameras becomes large, and it is difficult to connect the images at the joining portion without suppressing contradiction. Since the front ends of the cameras are made radially open, the camera lens layout becomes conspicuous and it is difficult to make compact the whole housing of a camera.
In a system using a hyperbolic mirror, an object image is focussed as a distorted image on the image pickup device, and converted into a normal image by image processing. Therefore, the pixel density of an image after conversion is not uniform over the whole area so that a portion greatly enlarged has coarse pixels and its image quality is degraded. There are less cases in which an image in a whole circumferential area becomes necessary for an application to a monitor camera and the like. If the monitor camera is mounted on a wall or the like, it is sufficient if the photographing field angle is about 120° to 180° in the horizontal direction. A monitor camera providing an image having a high resolution in this field angle range is desired. Such needs cannot be satisfied by a hyperbolic mirror system.
In a modeling system for generating a shape model of an existing substance described in Japanese Patent Application Laid-open No. H11-328444, a main camera and subsidiary cameras are disposed radially about a Z-axis, and their viewpoints (light reception axes) are set so that the viewpoints cross at one point (e.g., coordinate origin) on the Z-axis.
In a synchronous photographing method and system described in Japanese Patent Application Laid-open No. 2002-344800, three digital two-lens cameras each having two pairs of photographing units are used, and one camera is disposed just in front of the face of an object person, and the other two cameras are disposed at right and left positions slightly lower than the face.